Concept-Development 11-1

Name
Class
Date
Concept-Development
Practice Page
11-1
Torques
1. Felix Flex pulls the bar forward, rotates the
cam, and lifts the load. Two torques act on the
cam––the counterclockwise torque produced by
Felix’s pull P, and the clockwise torque produced
by the tension T that supports the load: Note
that although T stays constant, the torque is
not constant because of the variable lever
arm (hence the cam’s odd shape!).
Shown below is the cam in three positions,
A, B, and C. The lever arm (heavy dashed line)
is shown for position A.
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a. Draw the different-length lever arms for positions B and C.
(Take care that your lever arm is perpendicular to the line of action of T!)
b. In which position does the cam provide the longest lever arm?
c. In which position does T produce the greatest torque?
d. In which position does the cam provide the shortest lever arm?
e. In which position does T produce the least torque?
f. Which position requires the least pull P for Felix to rotate?
g. Which position requires the most pull P for Felix to rotate?
CONCEPTUAL PHYSICS
Chapter 11
Rotational Equilibrium
59
2. Pull the string gently and the spool rolls. The direction
of roll depends on the way the torque is applied.
In (1) and (2) below, the force and lever arm are shown
for the torque about the point where surface contact
is made (shown by the triangular “fulcrum”). The lever
arm is the heavy dashed line, which is different for each
different pulling position.
a. Construct the lever arm for the other positions.
b. The lever arm is longer when the string is on the (top) (bottom) of the spool spindle.
d. For the same pull, rotational acceleration is greater when the string is on
the (top) (bottom) (makes no difference).
e. At which positions does the spool roll to the left?
f. At which positions does the spool roll to the right?
g. At which position does the spool not roll at all?
h. Why does the spool slide rather than roll at this position?
3. We all know that a ball rolls down an incline. But relatively few people know that the reason the
ball picks up rotational speed is because of a torque. In Sketch A, we see the ingredients of the
torque acting on the ball––the force due to gravity and the lever arm to the point where surface
contact is made.
a. Construct the lever arms
for positions B and C.
b. As the incline becomes
steeper, the torque
(increases) (decreases)
(remains the same).
CONCEPTUAL PHYSICS
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Chapter 11
Rotational Equilibrium
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c. For a given pull, the torque is greater when the string is on the (top) (bottom).